(>4 TIII; NERVOUS S^STKM AND ITS cu\si:i;\ ATION 



itself at this place. If the overlying hone is removed the 

 several divisions of the brain will appear somewhat as in 

 Fig. 11. The medulla widens from its hinder to its 

 forward limit and the eye is caught by a deep triangular 

 hollow, opening in its dorsal surface. The apex of the 

 triangle is pointed back toward the cord. This hollow is 

 one of the chambers of the brain, the fourth ventricle. 

 Unlike all the other parts of the series of cavities, this 

 one breaks the surface; the others are completely sub- 

 merged and to be seen only after dissection. A thin roof 

 which partially covers the fourth ventricle in life is almost 

 invariably stripped from it in exposing the brain. The 

 central canal of the cord opens out into the fourth ven- 

 tricle at its pointed posterior extremity. 



Forward of the medulla are found the two large, ovoid 

 bodies which, with the corresponding ventral structures, 

 are conveniently covered by the term niidltrnin. A channel 

 extends from the fourth ventricle through this region to 

 the cavities still farther forward; it is picturesquely named 

 the aqueduct of Sylvius. \Ye shall find the midbrain to be 

 relatively much less prominent in the higher forms than 

 in the frog. An alternative name for this region, apply- 

 ing more accurately to the dorsal eminences, is the optic 

 lobes. 



In front of the midbrain we find in the case of the frog 

 that by far the larger part of what remains is the c<r<l>runi 

 or the ccnlinil In-mixphcrcx. We yet distinguish, however, 

 a limited portion of the brain between these paired struc- 

 ture- and directly before the midbrain as the ih'i //<< /ihalo/i. 

 In the diencephalon the aqueduct of Sylvius expands into 

 a space which is quite narrow when measured from right 

 to left, but more extensive dorsoventrally. This is the 

 third ventricle. Kach of the cerebral hemispheres has 

 within it a cavity which is called a lateral ventricle. 

 The total count of four ventricles is arrived at by reckon- 

 ing these two lateral chambers, the third in the dien- 

 cephalon, and the fourth in the medulla. The lateral 

 ventricles communicate with the third by small passages. 



